Electrical resistance of pure metals at cryogenic temperatures is extremely low. This makes pure metals attractive as cryogenic conductors. Of all the pure metals, aluminum is particularly promising for a number of reasons. Primary among these reasons is the low density of aluminum which makes it an optimum choice for weight critical applications such as space based systems. Secondly, aluminum has very high electrical and thermal conductivity at cryogenic temperatures. Aluminum also can be more economically produced in a high purity form than the other conductor material most commonly used, copper. The third most important advantage aluminum possesses over copper is its behavior in a strong magnetic field (&gt;2 Tesla). In the presence of a strong magnetic field, the electrical resistance of most pure metals increases. However, in the case of aluminum, the resistance approaches a saturation value at high magnetic fields or in essence has a very small linear increase with field. See H. Nomura, M. Obata and S. Shimanoto, Cryogenics, Vol. II, No. 5, p. 396, 1971. In comparison, copper shows a strong increase in resistance with field.
High purity aluminum conductors have very favorable electrical properties at the boiling point of hydrogen, 20K, and hence they are excellent candidate materials for use at this temperature. The resistance of high purity aluminum at this temperature is 1/500th its resistance at room temperature. In addition, it has been shown that the advantage of reducing resistivity of a cooled high purity aluminum conductor exceeds the energy spent to obtain the low temperatures. See V. I. Gostishehev, "Cryogenic Conductor Made of High Purity Aluminum", Fiz. Met. Metalloved, Vol. 62, No. 2, p. 303, 1986. An important advantage of 99.999 percent pure aluminum occurs at 20K, where such conductors are ideally suited for space based applications on the basis of liquid H.sub.2 being used as a fuel source and being available for use as a cryogen. Additionally, aluminum conductors are preferred over conventional superconductors in certain applications. Conventional superconductors require liquid He (4.2K) for their operation and the equipment for liquefaction and handling of liquid He adds complication and weight to the overall system. The advantage of aluminum is particularly true for fast pulsed power devices where superconductors are inherently unstable.
Due to its low mechanical strength, high purity aluminum needs to be structurally supported to withstand the large electromagnetic forces generated in high current density devices. As indicated in the article of Gostishehev, in windings made of cryogenic conductors, mechanical stresses come from two sources: the interaction of the magnetic field with the flowing currents and residual stresses during cooling. The stresses due to magnetic forces can be very significant and the high plasticity of aluminum can result in severe permanent deformation. Besides causing physical damage to the windings, the plastic strain also increases the resistance of the conductor significantly. Hence, in order to overcome the negative effects of low strength, pure aluminum conductors have to be structurally reinforced by some means.
High purity Al conductor may be braided with a high strength material to provide mechanical support. A preferred approach with economic and technical benefits is to embed the Al in a high strength matrix and co-fabricate them. The matrix material supporting the conductor in the composite assembly must have high strength, good thermal conductivity to remove heat generated in the conductor due to passage of current, reasonably high electrical resistivity to minimize eddy current losses in the matrix and workability compatible with high purity aluminum. In the absence of a barrier, probably the most important requirement is that the alloying elements of the matrix must have very low diffusion rates in aluminum to prevent contamination of the high purity Al conductor during processing. This precludes the use of most commercial aluminum alloys.
Powder metallurgy (P/M) processed Al-Fe-Ce alloys provide a good combination of properties toward satisfying the above requirements for a matrix material. These alloys were actually designed for elevated temperature applications and utilize thermodynamically stable aluminides for dispersion strengthening. The alloying elements Fe and Ce are two of the slowest diffusing species in aluminum. Research efforts at the Aero Propulsion Laboratories, Wright Patterson Air Force Base, Dayton, Ohio have demonstrated the feasibility of co-extruding a multifilament composite conductor, consisting of Al filaments in an Al-Fe-Ce matrix. See C. E. Oberly et al., U.S. Pat. No. 4,711,825, 1985 December 08, and J. C. Ho, C. E. Oberly, H. L. Gegel, W. T. O,Hara, Y. U. R. K. Prasad and W. M. Griffith, "Composite Aluminum Conductors for Pulsed Power Applications at Liquid Hydrogen Temperatures," Fifth IEEE Pulsed Power Conference. Arlington, Va., 1985 June 11. While 4,711,825 mentions the concept of using wire drawing for manufacturing its composite conductors, we do not believe such to have been accomplished, at least not in a practical sense, prior to the present invention. Thus, there can be varying degrees of success in the drawing of wire. For instance, it is one thing to pull a material straight on a draw bench, and quite another accomplishment to be able to pull material essentially continuously using a draw block to accomodate the drawn material as it collects.